'Parliament can no longer block a no-deal Brexit, the health secretary has said.
During his bid for the Tory leadership, Matt Hancock said no deal was "not an available choice" to the next PM, as MPs "will never allow it to happen".
He told the BBC he had now changed his mind because they had a chance to block it in a series of votes last month, but failed to muster the numbers.
Jeremy Corbyn said Labour would do all it could to stop no deal.
But Downing Street insisted the UK would be leaving on 31 October "whatever the circumstances", whether a deal had been agreed with the EU or not.
"Politicians cannot choose which votes to respect. They promised to respect the referendum result and we must do," No 10 said.
Asked whether Mr Johnson would respect the outcome of votes in Parliament - including a vote of no confidence in his government - the spokesman refused to be drawn, calling it a "hypothetical" question.
MPs have repeatedly rejected the agreement Theresa May reached with Brussels, but have so far failed to coalesce around an alternative.
In a no-deal scenario, the UK would immediately leave the EU with no agreement about the "divorce" process, overnight exiting the single market and customs union - arrangements designed to help facilitate trade.
Opponents say it would damage the economy and lead to border posts between Northern Ireland and the Republic - but other politicians argue any disruption could be quickly overcome.
On 12 June, the Commons rejected a motion tabled by Jeremy Corbyn, which would have allowed MPs to take control of the parliamentary timetable in the autumn to stop no deal going through.
Mr Hancock told BBC Radio 4's Today programme it was that vote which prompted him to change his mind.'
Read more: Brexit: Parliament can't stop no deal, says minister
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